It has been demonstrated on numerous occasions throughout research that measures of aerobic fitness are closely related to soccer performance. As a result the assessment of aerobic fitness on a regular basis is important for monitoring the effectiveness of the physical training program and the preparedness of soccer players before competition (Castanga et al, 2006). Traditional means to measure ones aerobic fitness has been centred on the ‘aerobic capacity’ or ‘VO2max’ of an individual. What is generally seen as the gold standard test in measuring ones VO2max (maximum oxygen consumption) involves running til exhaustion on a treadmill in a laboratory environment. Within soccer, VO2max is one of the most influential factors in determining players exercise intensity (Chamari et al, 2004). O’Reilly & Wong (2012) states that high levels of aerobic fitness in elite soccer players is extremely important. Various investigations have shown significant decreases in high intensity activity towards the latter stages of a soccer match. Therefore, O’Reilly & Wong (2012) believe the knowledge of a soccer player’s aerobic capacity is a valuable resource for coaches when identifying prospective talent.
“A soccer match generally lasts for about 90 minutes, and recent evidence suggests that, for elite soccer players, the average oxygen uptake during a match, after allowing for factors such as dehydration, hyperthermia and mental stress, is around 70% of maximum oxygen uptake” (O’Reilly & Wong, 2012).
A recent study by Helgerud et al (2002) demonstrates that by a subject increasing their VO2Max by 11%, match intensity would increase by 5% and distance covered in a match by 1800 metres. Hoff et al (2002) as cited by Charmari et al (2004) states that an improvement in running economy could also contribute to an improvement in aerobic endurance. Brewer (2005) is also in agreement and describes how an improvement in a players VO2Max will lead to an improvement in performance and also the relative intensity that the player can work at. Leger & Lambert, (1982) states that in order for one to assess the maximal aerobic power of a large group of subjects, one has to rely on valid, reliable, safe and inexpensive tests. The need for expensive and sophisticated technical equipment to measure maximal oxygen uptake (VO2max) has encouraged exercise scientists to develop a range of field tests that attempt to predict VO2max (Davies et al, 2008).

One test that has previously been most often used to assess ones aerobic endurance is that of the 12-min run (Cooper 1968). An issue with this test however is that it is maximal from the beginning to the end of the 12-minute period. The test therefore depends on the motivation of the subject to run at an even pace from start to finish. This test is therefore contrary to the ramped nature of various other VO2max tests. More current testing methods tend to favour the use of multistage exercise tests performing repeated shuttles until exhaustion (Leger & Lambert, 1982).
The 20-m multistage fitness test (MSFT) is arguably the most widely used test of aerobic fitness in the United Kingdom and is employed by many agencies including schools, sports clubs and the armed forces (Davies et al, 2008). A study by Ramsbottom et al (1988) suggested that a progressive shuttle run test provides a more valid estimate of VO2max. The MSFT is a test that has been used for many years as a predictor of ones VO2max (Ramsbottom et al 1988 ; Leger & Lambert 1982). Chatterjee et al (2009) describes the MSFT as an excellent performance indicator of aerobic fitness, which requires many changes of direction. The 20-m shuttle run test offers the advantages of both requiring no sophisticated equipment and enabling the testing of many individuals at the same time. Chatterjee et al (2009) found that the application of the present form of the MSFT may be justified in the standard adult population. However, to better predict VO2max among youth players (12-16yrs) they developed a new equation based on their own research.
The test does have its limitations; the data produced relies purely on the motivation of the athlete alone to reach their optimum level of performance (Coulson & Archer, 2009). Also a study by Aziz et al (2005) cites Bangsbo and Lindquist (1992) and Oliveira et al (2001b) and suggests that measured VO2max, obtained during soccer-specific field tests, cannot fully explain or characterise soccer players’ intermittent endurance performance. O’Reilly & Wong (2012), also in agreement state that the MSFT although previously used within a soccer setting may not be ideally suited to assessing the training effects of elite level players. O’Reilly & Wong (2012) discuss traditional means of measuring aerobic fitness centring around VO2max, stating that this has been shown not to be soccer specific to match situation. This has therefore lead to an increase in tests being developed to recreate exercise patterns experienced within a match situation. Therefore, numerous soccer-specific simulations have been designed to recreate exercise patterns similar to those experienced during a match.

The Yo-Yo Intermittent Test is designed to replicate the demands of sports such as football where game play is not continuous. There are two variations of the intermittent test: The Intermittent Endurance Test and the Intermittent Recovery Test each with two levels of varying intensity. The Intermittent Endurance Test consists of similar running speeds to the endurance test but includes an additional 5 seconds (5m) active recovery period in between each 20m shuttle. The Intermittent Recovery Test is more intense with running speeds beginning at 10kph (level 1) and 13kph (level 2). In contrast, there is a longer active recovery of 10 seconds (10m) between shuttles to allow more recovery (Bangsobo et al, 1998). The Yo-Yo Intermittent Endurance Test (YIET) (Bangsbo, 1996) was also developed as a football specific measure of predicting VO2max. However Castagna et al (2006) found that the Yo-Yo Test’s relationship to predicting VO2max was not as strong as that of the 20-m MSFT. Metaxas (2005) also states that he Yo-Yo field tests tends to under estimate an athletes VO2max. However, the MSFT involves a more continuous-type running protocol while the YIET has a five second recovery period after each shuttle runs of 40 m. It was perceived that the inclusion of this short active recovery interval in the YIET replicates more closely the typical high-intensity intermittent running interval that is inherent in soccer match play (Bangsbo, 1996).
The Yo-Yo Intermittent Recovery Tests (Yo-Yo IR) have rapidly become some of the most extensively studied fitness tests in sports science. Due to their specificity and practicality the tests have also been widely applied in many team sports to assess players’ abilities to repeatedly perform high-intensity exercise (Bangsbo et al, 2008). Bangsbo et al (2008) also in agreement with Castagna et al (2006) found that the Yo-Yo tests are not reliable at predicting ones VO2max, however Bangsbo claims that this is to be expected as the Yo-Yo tests are also evaluating a response during anaerobic exercise and recovery.

Discussion
It is hard to draw a definitive conclusion as to which method of field-testing is the most appropriate when assessing a soccer player’s aerobic fitness. The extensive research within this report has produced much contradictory research and negative criticism of all field-testing methods of assessing aerobic fitness. It is concluded that the 20-m MST is the most accurate and reliable form of field-testing to predict ones Vo2max, however not in its original form (Chatterjee et al, 2009). For more accuracy it is recommend that appropriate regression equations be used relating to the tested population in question to correctly evaluate ones aerobic fitness in terms of predicted VO2max. VO2max can be predicted from shuttle run test scores, but not as indicated with the original test package. In order to obtain the true score, one must apply a regression equation (Cetin et al, 2005).
The aerobic capacity is often evaluated through various field and laboratory tests to determine the soccer player’s level of fitness. Although as highlighted within various research, the majority of these tests are executed with protocols based on continuous types of exercise. However, Metaxas et al (2005) correctly states that a test is more reliable and effective when it is specific to the exercise patterns. This has therefore lead to many European soccer teams use of the Yo-Yo field test in daily practice (Metaxas et al, 2005). Therefore it is found that the Yo-Yo Tests are a greater reflection of soccer specific fitness, the repeated intense exercise interspersed with regular rest periods. Bangsbo et al (2008) found that the Yo-Yo IR tests showed more considerable improvements in performance after a period of training when compared to that of VO2max improvements.
Although the Yo-Yo tests are criticised due to the unreliability of predicting ones VO2max, Bangsbo (1996) does state that it was never an intention of the test to predict ones VO2max. The test was purely concerned with performance and ones ability to repeatedly perform intervals over a prolonged period of time. Where in contrast the 20-m MST was originally intended to predict the participants VO2max (Lambert, 1982 : Ramsbottom et al, 1988). Overall when summarising the collective research it seems applicable to suggest that soccer teams should not discount either the 20-m MST or the Yo-Yo Tests but instead utilise both throughout the season. An initial 20-m MST Test could be performed in preseason the establish individuals predicted VO2max levels by using the appropriate regression equations. The Yo-Yo tests may then be used more favorably throughout the season to assess the individual’s soccer specific endurance and monitor levels of fitness both aerobically and an-aerobically (Aziz et al, 2005).

References:

Aziz, A.R., Tan, F.H.Y., & Teh, K.C., (2005), A pilot study comparing two field tests with the treadmill run test in soccer players, Journal of Sports Science and Medicine, 4.